Polyesters such as, for example, polyethylene terephthalate, polytrimethylene terephthalate and polybutylene terephthalate, generally referred to as “polyalkylene terephthalates”, are a class of important industrial polymers. They are widely used in fibers, films, and molding applications.
Polyesters can be produced by transesterification of an ester such as dimethyl terephthalate (DMT) with a glycol followed by polycondensation or by direct esterification of an acid such as terephthalic acid (TPA) with a glycol followed by polycondensation. A catalyst is used to catalyze the esterification, transesterification and/or polycondensation.
For example, polyester can be produced by injecting a slurry mixture of TPA and glycol at about 80° C. into an esterifier. Linear oligomer with degree of polymerization less than 10 are formed in one or two esterifiers at temperatures from 240° C. to 290° C. The oligomer is then polymerized in one or two prepolymerizers and then a final polymerizer or finisher at temperatures from 250° C. to 300° C. TPA esterification is catalyzed by the carboxyl groups of the acid.
Antimony is often used for polymerization or polycondensation reaction. However, antimony forms insoluble antimony complexes that plug fiber spinnerets and leads in fiber spinning to frequent shutdowns to wipe spinnerets clean of precipitated antimony compounds. The antimony-based catalysts are also coming under increased environmental pressure and regulatory control, especially in food contact applications.
Titanium catalysts can be used in the esterification, transesterification, and polycondensation reactions. However, the titanium catalysts tend to hydrolyze on contact with water forming glycol-insoluble oligomeric species, which lose catalytic activity. Polyesters produced from an organic titanate also generate yellow discoloration. Even water compatible titanates, such as titanium bis-ammonium lactate, bis-triethanolamine titanate or the titanium sodium citrate catalysts disclosed in EP 812818, when used as polyesterification catalysts, generate significant yellow discoloration in the resultant polymer. Similarly, WO 99/28033 discloses an organometallic compound for producing an ester. The organometallic compound comprises the reaction product of an orthoester of titanium, zirconium, or aluminum, an alcohol containing at least two hydroxyl groups, an organophosphorus compound, and a base. When used as polyesterification catalyst, however, it was found that the organometallic compound also generates undesirably significant yellow discoloration in the final product.
Therefore, there is an increasing need for developing a new catalyst that is efficient, produces a polymer with reduced color, exhibits good catalytic activity, does not result in plugging fiber spinnerets, and is environmentally friendly.
An advantage of the present invention is the polymer produced using the invention catalyst has improved optical properties (e.g., less undesirable color) compared to polymer produced using an organic titanate catalyst alone. Other advantages will become more apparent as the invention is more fully disclosed herein below.